The present invention relates to a manipulator. More particularly, the present invention relates to a manipulator for supporting and orienting the test head of an automatic test system.
Manufacturers of semiconductor chips use automatic test equipment. Semiconductor chips are tested as early as possible in the manufacturing process to avoid the cost of processing defective chips. Generally, the chips are tested while they are still part of the semiconductor wafer on which they are fabricated. To test chips in this form, probes must be placed directly onto the chip because leads through which the chip is normally accessed are not added until the chips are separated from the wafer and packaged.
The probes are attached to a portion of the automatic test equipment called a xe2x80x9ctest head.xe2x80x9d The test head is connected through a cable to a mainframe cabinet, which holds a major portion of the electronic circuitry needed to generate and analyze test signals. The test head is supported by a manipulator, which allows the test head to be moved.
To test a chip, the wafer is mounted in a device called a prober. The test head is then moved by a human operator to bring the probes into contact with the chip. The test head is secured in this position, and testing is performed. The process of moving and securing the test head in a position in which electrical contact can be made to the chip is called xe2x80x9cdocking.xe2x80x9d Once docked, the prober moves the wafer around to present different chips on the wafer for testing.
Various factors make the docking process extremely difficult. First, the semiconductor chip is very small, on the order of a fraction of a square inch. Hundreds of test signals must often be coupled to the chip. The contact points for these test signals must all fit onto the small chip, requiring them to be very small. The test head must be precisely positioned so that all of the probes line up with all of the contact points.
Achieving precise positioning of the test head is complicated because the test head can weigh hundreds of pounds. The weight is largely caused by the need to position some electronic circuitry as close as possible to the chip being tested. The weight makes it difficult to move the test head. In addition, the test head can put excessive force on the prober, causing the prober to deform. If the prober deforms too much, it is very difficult to achieve precise positioning of the probes relative to the chip in the prober.
A further complicating factor is that the cable connecting the test head to the mainframe cabinet is very heavy and exhibits hysteresis. A cable can weigh hundreds of pounds. The cable weight can create forces on the test head. Because the cable exhibits hysteresis, these forces can change when the cable is moved. Thus, the manipulator must precisely position the test head despite the changing forces.
A further complicating factor is that different probers are often used with the same automatic test equipment. Different probers hold the semiconductor chip in different configurations. The manipulator must be able to position the test head to dock regardless of the type of prober used. Sometimes the test head is positioned horizontally above the prober with the probes facing down. In other instances it is positioned horizontally below the prober with the probes facing up. For other probers, the test head is positioned vertically with the probes facing sideways. Still other probers require the test head to be positioned at some angle between horizontal and vertical.
Various design techniques have been employed to facilitate docking. High precision manufacturing techniques are employed to ensure there is as little slack as possible in the manipulator. Hard stops are built into the manipulator to help it return repeatedly to the same position. Also, the contact area between the prober and the test head is made as large as possible to average out any deflections.
To make positioning easier and to reduce deflection caused by the weight of the test heads, counter weight schemes have been proposed. Also to ameliorate problems caused by the weight of the test head, some manipulators include motors or other drive mechanisms. To reduce the effect of the weight of the cable, arrangements have been proposed to support the weight of the cable.
Despite use of these design techniques, current manipulators can not always easily dock a test head with a prober. Often, two people are needed to handle the test head to get it into position. When the test head appears to be docked, numerous adjustments are sometimes needed to make electrical contact to all of the test points on the integrated circuit chip.
A manipulator for a test head comprises a base and a tower having vertical tracks. The tower is mounted on the base. A vertical shaft is mounted on the base adjacent to the tower. An outer carriage is in engagement with the vertical tracks. The outer carriage is movable relative to the tower along a vertical axis of the vertical tracks. The inner carriage is in engagement with the vertical shaft. The inner carriage is moveable relative to the tower along a vertical axis of the vertical shaft. A spring is disposed between the inner carriage an outer carriage. The spring supports the outer carriage. An arm is coupled to the outer carriage. The arm is adapted for supporting the test head.